1. Field of the Invention
This invention relates, in general, to electrical apparatus and, more specifically, to the detection and location of the source of corona discharges within such apparatus.
2. Description of the Prior Art
Corona testing of high voltage, fluid-filled electrical apparatus, such as transformers, is desirable, as it may locate weak points or faults in the insulation system which may cause partial breakdown of the insulation. Corona testing thus may locate incipient faults which allow the apparatus to pass conventional tests, but which may cause complete breakdown of the insulation at some future time.
The corona test must not only indicate the presence of corona in the fluid-filled apparatus, but it should provide some means for pinpointing its exact location. A typical prior art corona testing method for detecting and locating corona used the electrical signal provided by the corona discharge and a signal responsive to the mechanical vibrations produced in the insulating fluid by pressure changes in the fluid induced by the corona discharge. The electrical signal is used to trigger the horizontal sweep of a cathode ray oscilloscope, and the mechanical vibrations are picked up by a suitably located mechanical to electrical transducer which provides an electrical output signal to the vertical deflection terminal of the oscilloscope. The time between the start of the sweep and the start of the vertical deflection indicates the distance from the corona discharge to the pickup of the transducer. Moving the transducer and repeating the sequence allows the approximate location of the corona discharge to be identified by triangulation.
As shown in U.S. Pat. Nos. 3,505,597, 3,612,992 and 3,728,619, all of which are assigned to the assignee of the present application, the acoustic sensors or mechanical to electrical transducers commonly used for the detection and location of internal corona discharges are mounted on the exterior surface of the steel tanks of the fluid-filled electrical apparatus. The acoustic sensors are mounted externally on the tank due to the high voltages involved during testing and actual operation of the electrical apparatus which discourages the internal location of such sensors which are at ground potential. Depending upon the thickness of the steel tank wall, the loss of sensitivity between an external and internal sensor could be as high as 10 to 1, with attenuation and reflections by the electrical apparatus structure causing further signal losses and ambiguities. Obviously, internal mounting of the acoustic sensors is desirable due to greater sensitivity and discharge location accuracy; however, it is hazardous. Thus, it is apparent that the accuracy of detecting and locating the source of corona discharges within electrical apparatus depends to a large extent upon the strength of the pressure signal received by the acoustic sensors.
It would be desirable to provide improved corona detection means for electrical apparatus which has greater sensitivity and corona discharge location accuracy. It would also be desirable to provide corona detection apparatus which may be inserted into the tank of an electrical apparatus without creating electrical stress problems due to the high voltage associated with such electrical apparatus. It would also be desirable to provide corona detection means for an electrical apparatus which may be permanently installed within such apparatus to thereby act as an incipient fault detector.
Waveguides have been previously used to transmit acoustic vibrations in the form of acoustic energy between two locations in electrical apparatus as shown in co-pending application Ser. No. 858,054, filed Dec. 6, 1977, in the names of R. Harrold and A. Bennett. However, the waveguides have been positioned such that the acoustic vibrations enter the waveguide substantially perpendicular to one end thereof and are transmitted through the waveguide to the other end. When used in this manner, many acoustic waveguides, each having one end disposed at a precise location within the electrical apparatus, are necessary to provide accurate detecting and locating of corona discharges throughout the electrical apparatus.